Cutting insert having a flat edge surface for supporting the insert in a tool holder

ABSTRACT

A cutting insert is produced by form-pressing and sintering of powder. The insert comprises a top chip surface, a bottom surface, and at least one edge surface extending between the top and bottom surfaces. The edge surface includes at least one relief surface portion which intersects the top surface at an acute angle to form therewith a cutting edge. A grinding operation is performed on the edge surface, but only along a lower portion thereof disposed below the relief surface portion, i.e., only along an area which is to be placed against a locating surface of a machine tool during a cutting operation.

This application is a Divisional of application Ser. No. 08/230,209,filed Apr. 20, 1994 which, in turn, in a Divisional of application Ser.No. 08/004,302 filed Jan. 14, 1993, U.S. Pat. No. 5,365,805.

BACKGROUND OF THE INVENTION

The present invention relates to a cutting insert for chipbreakingmachining tools, in particular for milling tools such as facemillingtools, and to a process for making the insert.

Such an insert is typically manufactured by form-pressing and sinteringof an insert-forming powder material to form a body which comprises atop chip surface, a suitably planar bottom surface that can rest upon aseat surface of the machining tool, and at least one edge surfaceextending between the top and bottom surfaces. The edge surface, whichcan be placed in abutment with at least one cooperating side abutmentsurface of the tool, is generally inclined at an acute angle withrespect to the chip surface and at an obtuse angle with respect to thebottom surface, whereby a cutting edge is formed along the intersectionof the chip surface and the edge surface, adjacent to which cutting edgethere are one or several relief or clearance faces.

For the manufacturing of such cutting inserts, in particular indexablecutting inserts, of hard metal, a direct-pressing method is frequentlyused, in which a hard metal-forming powder first is formed to theintended shape in a suitable pressing die and then given the finalstrength and size by sintering in an oven at a temperature above 1000°C. The pressing operation as such has been further developed over theyears and is today so advanced that it enables the formation of thecutting edges and adjacent chipforming faces and possible reinforcingfaces with great dimensional precision. However, during the sinteringoperation a shrinkage takes place (usually amounting to about 18% of theoriginal length in each dimension) and due to this, the cutting insertloses some of its original precision.

For some types of machining, e.g., some sorts of facemilling, therequirements of form and dimensional precision have become more rigorousover the last years. Particularly insert geometries with a positivecutting edge require a very high degree of dimensional accuracy toguarantee a satisfactory function at small tooth feeds. These precisionrequirements have up to now been met by so-called contour grinding,which consists of after-grinding the surface(s) adjacent to theindividual cutting edge in one step after sintering. However, a seriousdisadvantage of such contour grinding is that it causes modifications inthe micro-geometry of the insert, i.e., in the surface structure of theinsert's edge-shaping parts after a surface treatment such as blasting,face grinding or deposition of a hardness-improving surface layer, whichis usually done as soon as possible after the sintering has beenfinished. Thus, the width of existing negative reinforcing surfaces isaltered, as well as the distance from the cutting edge to thechipforming surfaces. In practice, this means that the chipformingability and the cutting performance of the cutting insert are diminishedand that its strength and life are reduced.

SUMMARY OF THE INVENTION

An object of the present invention is to remove the above mentioneddisadvantages by eliminating every form of after-grinding in theimmediate proximity of the cutting edge(s). Thus, a primary object ofthe invention is to produce a cutting insert whose working dimensionscan be established with high accuracy without necessitating anyafter-grinding of the cutting edges in question. A further object of theinvention is to enable a simple and rational production of such inserts.

In a process aspect of the invention, a powder is pressed to form aninsert body having a top chip surface, a bottom surface, and at leastone edge surface interconnecting the top and bottom surfaces. The edgesurface includes a relief surface portion intersecting the top surfaceat an acute angle to form therewith a cutting edge. That press-formedbody is then sintered. Finally, the edge surface is ground, but onlyalong a lower portion thereof disposed below the relief surface portion.

That lower portion of the edge surface is, during a cutting operation,placed against a locating surface of a machine tool. By subjecting onlythat lower surface portion to a grinding operation, the insertdimensions assume a much greater degree of precision while avoiding theaforementioned problems resulting when the entire edge surface and/ortop surface are subjected to grinding.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will be described below indetail with reference to the accompanying drawings in which:

FIG. 1 is a simplified top perspective view of a cutting insertaccording to the present invention;

FIG. 2 is a transparent top perspective view of the same cutting insert;

FIG. 3 is a side view of the cutting insert in connection with aschematically shown grinding tool;

FIG. 4 is an end view of a facemilling tool equipped with cuttinginserts according to the invention and illustrating the positioning ofthe inserts on the facemill; and

FIG. 5 is a side view of the facemill of FIG. 4.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

In FIGS. 1-3, a cutting insert 1 of square basic shape is shown. Thecutting insert comprises a top chip surface 2 and a suitably planarbottom surface 3 which is substantially parallel with the plane P₂ ofthe chip surface 2. In this square embodiment four identical edgesurfaces 4A, 4B, 4C and 4D extend between the chip surface 2 and thebottom surface 3. The cutting insert has a positive geometry, meaningthat the insert's edge surfaces 4 lie in planes that on the one handform an obtuse angle with the plane of the bottom surface 3 and on theother hand form an acute angle with the plane of the chip surface P₂.

Disposed between adjacent edge surfaces are corners 5, 5', 5" and 5"'.In the area between the chip surface 2 and one of the edge surfaces 4Aare disposed two cutting edges, viz. a main cutting edge 6 and asecondary cutting edge 7. In an analogous manner, between chip surface 2and another edge surface 4B are disposed a main cutting edge 6' and asecondary cutting edge 7', similar pairs of cutting edges 6", 7" and6"', 7"' being formed in the transitions between the chip surface andeach one of the edge surfaces 4C and 4D. Each main cutting edge 6 formsa certain angle with the secondary cutting edge 7 as the surface 2 isviewed in plan. In practice, this angle should lie within the range of0.5°-4°.

Along a certain part of each edge surface, the cutting insert 1 has aplanar relief surface 8, which extends along a substantial extent of therespective main cutting edge 6 and whose width or height increasestoward a corner located adjacent that main cutting edge. For example, atthe edge surface 4A, the width of relief surface 8 gradually decreasesin the direction from the corner 5"' towards the corner 5. This reliefsurface 8 is oriented in a plane substantially perpendicular to theplane P₂, which is clearly shown in FIG. 3. Along a dividing or junctionline 9 this first relief surface 8 joins a second relief surface 10,whose width decreases in the direction from the corner 5 towards thecorner 5"'. This second relief surface 10 is inclined at an acute angleγ in relation to the plane P₂ of the chip surface, for instance an anglewithin the range of 65°-75°, suitably around 70°, as shown in FIG. 3.The top surface 2 includes an upper reinforcing land 11 joining thecutting edges 6, 7 and having a substantially constant width, at leastalong the main cutting edge 6. This land preferably lies in the plane P₂and joins an intermediate surface portion 11A which, in turn, joins achipbreaking portion 2A of the top surface 2. The latter can includemany different chip breakers.

In the center of the cutting insert there is a hole for the applicationof a suitable fastening means.

The cutting insert described so far has been previously described inSwedish Application 9003827-4 and European Publication No. 489 702.

Besides the two relief surfaces 8, 10, each edge surface 4A-4D accordingto the present invention also comprises a third surface portion 12 whichis formed by grinding of the cutting insert. In FIG. 3 it isschematically illustrated how this third surface 12 can be produced byapplying a grinding tool 13 against the edge surface in question. InFIG. 3 the grinding tool 13 is in contact with surface 4A. The cuttingedge 6" on the diametrically opposed side 4C functions as an abutmentfor positioning the insert 13 during the grinding operation. Morespecifically, the grinding tool is brought to such a depth that aprecise dimension A is attained between the cutting edge 6" and thediametrically opposed surface 12. This is important, since the groundsurface 12 will abut a locating surface of a machine tool in order toposition the diagonally opposite cutting edge during a cuttingoperation.

The inclination angle θ of the ground surface 12 in relation to theplane P₂ of the chip surface is in practice somewhat smaller than thepreviously mentioned inclination angle γ of the other relief surface 10.In practice, the difference between those angles should lie within therange 1°-6° and preferably about 4°. Thus, if the angle γ is 70°, thenthe angle θ should amount to about 66°. A junction edge 14 is thusformed between the two surfaces 10 and 12. In practice, this edge 14should extend parallel to the bottom surface 3, giving the surface 12 aconstant width W along its whole length. In practice, the width of thissurface 12 is about half of the insert's thickness T, although it couldalternatively be less or greater. However, the width W of the groundsurface 12 should always amount to at least 40% of the insert'sthickness T.

Naturally, the grinding operation is resource-demanding in that itrequires time and energy. In order to reduce time and energy consumptionto a minimum, the cutting insert has been provided with a preferablycentrally placed recess 15 on each edge surface 4A-4D during theform-pressing step. This recess divides the ground surface on each sideof the cutting insert into two surface portions 12₁ and 12₂. Inpractice, the length of the recess 15 can be as much as 25-35% of thewhole length L of the ground surface 12, so that the total surface areaof the surface portions 12₁ and 12₂ is about 75-65% of the area that theground surface would have had if it had not been interrupted by therecess 15.

While the surfaces 8 and 10 next to the cutting edges are kept in thesame shape as formed by form-pressing and sintering steps, the surfaces12 are produced by grinding. This enables a very high degree ofdimensional accuracy to be attained in that the tolerance of theafore-described dimension A between the individual grinding surface anda diametrically opposed cutting edge (i.e., the variance of thatdimension A from a desired value) will lie within a very small range,i.e., the range 1-20 μm, preferably 1-10 μm. The individual groundsurface 12 serves as an abutment surface in connection with a sideabutment surface of the machining tool for positioning the insert duringcutting.

FIGS. 4 and 5 illustrate a facemiller 15 equipped with a number ofcutting inserts 1 according to the invention (although the cuttinginserts in FIG. 5 are shown with a differently shaped chip surface thanthe cutting insert shown in FIG. 1). The cutting inserts are placed intorecesses of the milling cutter 15 in such a way that the milled angle inthe working piece will be 90°. In each individual recess of the millingcutter there are three separate abutment surfaces, viz. a bottomabutment or seat surface 16 against which the bottom surface 3 of thecutting insert is placed, a first side abutment surface 17 and a secondabutment surface 18 against which two of the edge surfaces 12 of thecutting insert are placed, while the cutting edges of the other two edgesurfaces are in a working position. Generally, the geometry is such thatthe radial rake angle α is negative and the axial rake angle β ispositive. Decisive for the machining accuracy is on the one hand theradial dimension B between the rotation axis of the milling cutter and aperipherally situated main cutting edge 6 on the individual cuttinginsert, and on the other hand the axial dimension C between the upper,planar surface 19 of the milling cutter and the secondary cutting edges7 of the individual cutting insert. These dimensions B and C are, ofcourse, dependent upon the dimension A between the individual cuttingedge and the diametrically opposed abutment surface 12 which abutseither of the surfaces 17 and 18. By making this abutment surface 12 inthe form of a ground surface, not only the benefit of high dimensionalprecision is realized, but also the basic advantage that themicro-geometry of the cutting insert can be maintained unchanged aftersintering and a possible surface treatment. In this way, inter alia, thereinforcing faces 11 which were produced during the form-pressing andsintering steps, can keep their original width, and the chip breakers inthe chip surface 2 can keep their positions in relation with the cuttingedges.

The invention is naturally not restricted to what has been describedabove or to the embodiment illustrated in the drawings. Thus, it is alsofeasible to use the inventive concept for cutting inserts with anotherpolygonal basic form than the square shape, e.g., a triangular shape. Itis even feasible to produce circular cutting inserts with the lowerportion of the sole circumferential edge surface being ground. Further,the cutting insert according to the invention can also be applied toother chipbreaking machining tools than just facemillers.

Moreover, it is pointed out that the cutting insert according to theinvention need not necessarily have two cutting edges 6, 7 and twodistinct relief surfaces 8, 10 separated by an inclined juncture edge 9.Rather, it is also possible to produce the cutting insert with only onerelief surface and one single cutting edge along each side (in the caseof a polygonal cutting insert), whereby that relief surface joins aground surface having a smaller angle in relation with the chip surfacethan the relief surface.

Furthermore, the design of the chip breakers on the top surface of thecutting insert can vary quite considerably, as well as the form and thedimensions of all possible reinforcing faces.

The inventive concept is also applicable on cutting inserts made ofother materials than hard metal, as long as a powder is form-pressed andsintered.

What is claimed is:
 1. A cutting insert for chipbreaking machining, saidinsert comprising a pressed and sintered powder body having a top chipsurface, a bottom surface, and at least one edge surface interconnectingsaid top chip surface to form therewith a cutting edge; said edgesurface also intersecting said bottom surface; said edge surfaceincluding a recess having an upper end spaced below said cutting edge,and first and second sides extending downwardly from said upper end; afirst insert-supporting surface portion extending from said first sideof said recess in a direction away from said second side; a secondinsert-supporting surface portion extending from said second side ofsaid recess in a direction away from said first side; each of said firstand second insert-supporting surface portions being flat and coplanarand extending downwardly to said bottom surface; each of said first andsecond insert-supporting surface portions forming an identical acuteangle with a plane of said top chip surface.
 2. The cutting insertaccording to claim 1, wherein said recess has a length extendingparallel to said cutting edge, said length being 25-35% of a combinedlength of said recess and said first and second insert-supportingsurface portions as measured along said bottom surface.
 3. The cuttinginsert according to claim 1, wherein said edge surface further includesat least one clearance surface disposed between said first and secondinsert-supporting surface portions and said cutting edge and forming afirst acute angle with a plane of said top chip surface said first acuteangle being larger than a second acute angle formed by each of saidfirst and second insert-supporting surface portions with said plane. 4.The cutting insert according to claim 1, wherein said recess extendsdownwardly and intersects said bottom surface.
 5. The cutting insertaccording to claim 1, wherein said edge surface comprises one of aplurality of identically configured edge surfaces of said insert; saidedge surfaces intersecting to form corner portions of said insert. 6.The cutting insert according to claim 5, wherein there are four of saidedge surfaces.